The present invention relates to a sensor for sensing proximity or touch of an object such as a finger and to a method of calibrating a proximity or touch sensor.
A proximity or touch sensor may function by detecting a change in capacitance of a sensing capacitor of the sensor due to contact or proximity of the object. The change in capacitance may in turn be detected by detecting a change in frequency of an oscillating signal generated by an oscillator circuit comprising that sensing capacitor. Such an oscillator circuit may, for example, include a resistive-capacitive (RC) oscillator circuit in which the oscillating frequency decreases with increasing capacitance of the sensing capacitor.
A multi-channel capacitive proximity or touch sensor may include multiple sensing capacitors spatially distributed on the sensor. To achieve simplicity and lower costs, a change in capacitance of each multiple sensing capacitor may be detected by the same processor. The processor may be sequentially coupled to each sensing capacitor for a short period of time to detect the touch or proximity. While it may lower costs, use of a single processor in a multi-channel sensor may require that each sensing capacitor has the same or similar static or parasitic capacitance (capacitance when untouched). Otherwise the different sensing capacitors may cause a large variation of oscillation frequencies, some of which may be too low or too high for the processor to accurately determine any relative change in oscillation frequency. For at least these reasons, a multi-channel sensor may have little flexibility in using sensing capacitors of different capacitance. This lack of flexibility means that the different sensing capacitors may not be sized or shaped differently or too differently. This lack of flexibility may also make it difficult to improve spatial resolution or accuracy of the multi-channel sensor.